An important consideration in web manufacturing and other applications utilizing a web is the assurance of dirt-free web product. Applications such as photographic film manufacturing require cleaning and detecting particulate contamination on the web in order to minimize defects and imperfections in the web, which is important in the manufacture of products such as emulsion-coated photographic films. The detection of low level contamination is particularly important in web manufacturing applications having a fast-moving web, since the failure to detect contamination can result in having to scrap a large quantity of product.
U.S. Pat. No. 3,588,513 discloses an apparatus for photoelectric inspection of sheet materials. The apparatus comprises a light beam emitting means, for directing a light beam against the surface of the sheet material, and a means for detecting the reflection of the light from defective parts on the sheet material and for generating an electric signal corresponding to the defects of the sheet material. A disadvantage of the apparatus is that it looks directly at the sheet material and thus has a low sensitivity for measuring low levels of particulate contamination.
U.S. Pat. No. 4,019,066 discloses a method and apparatus for measuring the surface roughness of a material. The method comprises reflecting light off a moving web, measuring the reflected light, converting the collected light to electrical signals, and analyzing a direct and an alternating component of the signals to derive an index of surface roughness. Again, the apparatus has the disadvantage of looking directly at the web, rendering its sensitivity low for low level contamination. The reference also discloses that particles on the web surface having lower brightness or reflectivity can be registered by the apparatus and in certain, unspecified circumstances distort the roughness measurements.
U.S. Pat. No. 4,053,237 discloses a method of testing the surface texture of a steel chill roll. A collimated light beam is focused on the roll surface, a glossmeter is positioned on the surface, and responses are correlated to predict an expected surface texture of plastic film cast against the surface. A disadvantage is that the method applies to off-line quality testing of a roll surface texture and is unfeasible for on-line measurement of texture or contamination of a roll surface.
It is an object of the invention to provide an apparatus and method for measuring particulate contamination that is capable of measuring low levels of contamination on a web, that can be used on-line, and that is also responsive to the presence of low level contamination on a fast-moving web.